In the alkaline refining of polymeric carbohydrates and particularly in the processing of natural products containing cellulose, hemicellulose is dissolved into the caustic solution. As used herein, the term hemicellulose should be understood to include all those organic compounds which during the course of the alkali treatment are dissolved into such a caustic solution. The hemicellulose-containing caustic liquors resulting from such treatment can usually be recirculated into the process, but the content of hemicellulose in the circulating liquor gradually increases until finally it reaches a state of equilibrium. At this stage the amount of hemicellulose entering the refining process in the alkali solution equals that removed by the refining treatment. This means that the amount of hemicellulose introduced into the process in the starting material equals that removed from the process with the refined product, with the result that refining is frustrated and the removal of hemicellulose in fact does not occur. Thus, in order for refining of the pulp to occur, hemicellulose-containing liquor must be continuously removed from the process by one means or the other, and the removal must be compensated for by the addition of a like amount of fresh alkali. The removed solution can be treated to regenerate the alkali present therein, but otherwise it must be destroyed or used for other purposes.
This process is particularly important, for example, in the manufacture of viscose products and cellulose derivatives from pulp, where, in most cases, the first step is mercerisation; that is, steeping treatment of the cellulosic material with concentrated alkali solutions which usually contain more than 17% sodium hydroxide. This treatment results in the dissolution of a major proportion of the hemicelluloses and degradation products of the pulp so that a substantially purified cellulose remains. Depending on the degree of refining of the pulp used as raw material in the viscose manufacture, the alpha cellulose content generally varies within the range 89-98% by weight, while the alpha level of unrefined bleached pulps is 85-89% by weight. If the pulp is unrefined or refined to only a small degree, its hemicellulose content is relatively high, and a greater amount of hemicellulose will become dissolved in the steeping liquor during the steeping treatment. To avoid high hemicellulose content in the steeping liquor, most manufacturers of viscose products in the past found it necessary to purify their steeping liquors by dialysis. However, such processes not only require a considerable initial investment and high operating costs, but also yield a large amount of rather dilute NaOH solution that must be concentrated for reapplication, mainly by removal of water by evaporation. Today, because of these disadvantages, only a few manufacturers of viscose products apply dialysis in the purification of their steeping liquors. In the usual case, the hemicellulose-containing liquor is recirculated in the process with only a rather small amount of alkali being taken from the steeping liquor system for use at a later stage (after removal of the fiber fragments and suspended fibers) for dissolution of xanthate which is formed by treating the alkali cellulose with carbon disulfide. Accordingly, after equilibrium has been attained, all of the hemicellulose of the pulp and the low-molecular substances formed during the course of ageing end up in the viscose.
As previously indicated, at equilibrium, the hemicellulose content of the steeping liquor will be proportional to the degree to which the pulp has been refined. For example, the use of a pulp having an alpha-cellulose content of 93-94% will result in a liquor having a hemicellulose content of about 20-25 g/l or even somewhat higher, whereas the utilization of a pulp having an alpha level of approximately 90% yields a liquor having a hemicellulose content of about 50 g/l. Such a high content of hemicellulose in the steeping liquor exercises a detrimental effect on the mercerising power of the alkali with the result that an unduly high amount of carbon disulfide is required in xanthation. Even then, the quality of the viscose may be poor. Thus, preparation of acceptable viscose by most conventional methods has been possible only from fully bleached, more-or-less refined, pulps.
In a departure from such methods, the recently developed so-called SINI process (disclosed and claimed in U.S. Pat. Nos. 3,600,379 and 3,728,330) in which the alkali cellulose is subjected to resteeping and repressing prior to xanthation by application of a steeping solution with a NaOH content lower than 15%, has proved very efficient and makes it possible to manufacture high quality viscose even from unrefined pulp. However, where unrefined pulp is employed in the process, the concentration of hemicellulose in the circulating steeping liquor will become so high that utilization of unrefined pulp is impracticable unless the hemicellulose is removed from the steeping liquor. It can be estimated that the content of hemicellulose in the two steeping liquor systems of the SINI process will rise to 100 g/l, if unrefined pulp is utilized as raw material.